What is the purpose of working for reward?
The purpose is to enhance God's work and Kingdom and to bring honor and glory to His Name. Believers already have salvation by grace through faith, so they do not work for salvation; after entering that salvation experience, they work for reward, running the race to obtain the prize.
Les explains that most of Christendom thinks people work to attain salvation, but salvation is absolutely not by works. It is all of grace by faith plus nothing. However, once a person has entered the glorious salvation experience, God expects that believer to work for reward. The purpose of that work is to enhance His work, to enhance the Kingdom, and to bring honor and glory to His Name. Les points out that God is still the Sovereign God and believers are still His little worker bees. Paul uses the analogy of the Olympics in I Corinthians 9:24: people run in a race, but one receives the prize. Believers are to run that they may obtain, not for salvation, which they have already attained by faith.